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Vacuum and compression waveforms
Posted to Technical Theory Forum on 5/31/2014 23 Replies

Well I took a little time to experiment with vacuum and compression waveforms on my personal car. I used a homemade vacuum transducer using radio shack parts, a Pico pressure transducer and a PV-350 for the Verus screen shots. I was wondering if I could tell it the valves needed adjusting or not. So I took a compression waveform and a vacuum waveform while doing different things with the valve adjustment. First I tightened the valve lash to zero on #4 Cylinder exhaust valve. This is what I got: [2000 Honda Accord EX, Engine/Propulsion Waveform]. Notice the pressure spike in the intake when the intake opened and the valve overlap.

I then wondered what it would look like with all the exhaust valves tight. This is What I got: [2000 Honda Accord EX, Engine/Propulsion Waveform]. Notice the 5 volt swing of the Vacuum transducer. Also notice the higher running compression reading of a little over 90 psi. Extra exhaust volume raised the pressures.

I then tightened the intake and exhaust valves to zero lash. This is what I got: [2000 Honda Accord EX, Engine/Propulsion Waveform]. Notice it had about a 5 volt swing of the Vacuum transducer. The engine would barely idle with the valve like that. The running compression was about 90+ PSI.

I then Adjusted the valves to spec. This is what I got: [2000 Honda Accord EX, Engine/Propulsion Waveform]. Notice that the vacuum transducer only had about a 3 volt swing. Running compression was only about 65 psi.

If all the valves are the same out of spec, the only way you will know for certain there is a problem is if you have know good vacuum waveforms. If one valve is bad with adjustment or leaking I think you would be able to see it and with an overlay to determine which cylinder.

I thought it would be great on the Honda V6 engine to see if it needs a valve adjustment when they have misfire codes but they run good in the shop. I can upload the whole file if someone wants to look at it.

Robert from California

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car Vehicle Data

2000 Honda Accord EX 2.3L

Engine2.3 L
Trans5-speed Standard